I was just going through my BD copy of Iron Man and I think I know which scene you are talking about now. It occurs at approx 1:16. There is defintely some demanding low frequency content present in that scene and the one right after with the explosion. If you can isolate the noise to just the bottom two drivers which handle the bass reproduction on the M80's, the drivers are probably having a hard time reproducing those scenes without distortion.
If you can isolate the sounds to the entire speaker, the amp clipping the signal is more likely. I suspect it is the former and not the latter.
